Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are two commercially important marine fishes impacted by both overfishing and climate change. Increasing ocean temperatures are affecting the physiology of these species and causing changes in distribution, growth, and maturity. While the physiology of cod has been well investigated, that of haddock has received very little attention. Here, we measured the metabolic response to increasing temperatures, as well as the critical thermal maximum (CT_max), of cod acclimated to 8 and 12 °C and haddock acclimated to 12 °C. We also compared the swimming performance (critical swimming speed,U_crit) of cod and haddock at 12 °C, as well as theU_critof 12 °C-acclimated cod acutely exposed to a higher-than-optimal temperature (16 °C). The CT_maxfor cod was 21.4 and 23.0 °C for 8- and 12 °C-acclimated fish, respectively, whereas that for the 12 °C-acclimated haddock was 23.9 °C. These values were all significantly different and show that haddock are more tolerant of high temperatures. The aerobic maximum metabolic rate (MMR) of swimming cod remained high at 16 °C, suggesting that maximum oxygen transport capacity was not limited at a temperature above optimal in this species. However, signs of impaired swimming (struggling) were becoming evident at 16 °C. Haddock were found to reach a higherU_critthan cod at 12 °C (3.02 vs. 2.62 body lengths s⁻¹, respectively), and at a lower MMR. Taken together, these results suggest that haddock perform better than cod in warmer conditions, and that haddock are the superior swimmer amongst the two species.

Introduction: In order to ensure lipid quality of cultured fish and an environmentally sustainable production, new alternative sources of EPA and DHA are needed to replace traditional lipid sources, such as fish oil.Methods: Different combinations of alternative marine lipid sources rich in n-3 LC-PUFA available in the market were herein evaluated to replace traditional fish oil (sardine oil) in diets for European sea bass (Dicentrarchus labrax). A commercial-type diet (CTRL), containing 1.6% of EPA + DHA, 5% sardine oil and 8% rapeseed oil was used as a negative control. Another diet (SARDINE) formulated with 8.5% sardine oil, 4.5% rapeseed oil and 2.5% EPA + DHA was used as the positive control. Three experimental diets were formulated to completely replace sardine oil with alternative sources, targeting approximately the same EPA + DHA level as the positive control: the SALMON diet contained 9.9% salmon by-product oil mixed with 3.1% of an algal oil rich in EPA and DHA, while the ALGARAPE and the ALGASOY diets included 4.4% of the algal oil and 8.6% of either rapeseed or soybean oil, respectively. A sixth diet (ALGABLEND) was formulated to partially replace sardine oil with salmon by-product oil and rapeseed oil, balanced with 2% of algae biomass. The experimental diets were hand-fed to 118 g fish for 54 days. Results: All diets were well-accepted by fish and no significant differences were found in feed efficiency, growth performance, somatic indexes or whole body composition among treatments. At the end of the trial, regardless the dietary EPA + DHA level, all fillets contained more than 250 mg of EPA + DHA per 100 g fresh weight, meeting EFSA recommendations for cardiovascular risk prevention for European adults (> 250 mg day ^-1). Discussion: Overall, this study demonstrated that combining expensive sources of n-3 LC PUFA (Veramaris® or Algaessence Feed™ with low-priced sustainable oils (salmon by-products oil or vegetable oils) allows fortifying European sea bass flesh with EPA and DHA, without major textural changes. This approach is a successful strategy for mitigating the negative effects associated with the high inclusion of vegetable oils. However, the retention of n-3 LC-PUFA in muscle was not significantly increased, suggesting that there is a maximum dietary threshold beyond which β-oxidation might be promoted, and hence there is no advantage in increasing the dietary level of these fatty acids in European sea bass diets.

Author's accepted version (postprint).Available from 06/03/2018.The full expression of growth potential in fish larvae largely depends on an efficient protein utilisation, which requires that all the indispensable amino acids (IAA) are provided at an optimum ratio. The effect of supplementing a practical microdiet with encapsulated crystalline-AA to correct possible IAA deficiencies was evaluated in Senegalese sole larvae. Two isonitrogenous and isoenergetic microdiets were formulated and processed to have approximately the same ingredients and proximate composition. The control diet (CTRL) was based on protein sources commonly used in the aquafeed industry. In the supplemented diet (SUP) 8% of an encapsulated fish protein hydrolysate was replaced by crystalline-AA in order to increase the dietary IAA levels. The microdiets were delivered from mouth-opening upon a co-feeding regime until 51 days after hatching (DAH). The larvae capacity to utilise protein was evaluated using an in vivo method of controlled tube-feeding during relevant stages throughout development: pre-metamorphosis (13 DAH); metamorphosis climax (19 DAH) and metamorphosis completion (25 DAH). Somatic growth was monitored during the whole trial. A possible effect on the regulation of muscle growth was evaluated through muscle cellularity and the expression of related genes (myf5, myod2, myogenin, mrf4, myhc and mstn1) at metamorphosis climax (19 DAH) and at a juvenile stage (51 DAH). The SUP diet had a negative impact on larvae somatic growth after the metamorphosis, even though it had no effect on the development of Senegalese sole larvae capacity to retain protein. Instead, changes in somatic growth may reflect alterations on muscle growth regulation, since muscle cellularity suggested delayed muscle development in the SUP group at 51 DAH. Transcript levels of key genes regulating myogenesis changed between groups, during the metamorphosis climax and at the 51 DAH. The group fed the SUP diet had lower dnmt3b mRNA levels compared to the CTRL group. Further studies are needed to ascertain whether this would possibly lead to an overall DNA hypomethylation in skeletal muscle